A. Field of the Invention
The present invention relates to a switching device that can be used for driver switching devices of a display panel using an organic EL or a liquid crystal, and for high density memories.
B. Description of the Related Art
Recently, the performance of organic electronic materials has made spectacular advances. Some low dimensional conductors, such as charge transfer complexes, exhibit a distinctive property, for example, metal-insulator transition, and have been studied for application to driver switching devices of organic EL display panels or to high density memories.
Organic bistable materials are drawing attention as materials that can be applied to the switching devices. An organic bistable material is an organic material showing a so-called non-linear response. A switching phenomenon is observed in which the current in a circuit abruptly increases at a certain voltage when an increasing voltage is applied to the material.
FIG. 7 shows an example of voltage-current characteristic of an organic bistable material exhibiting such switching behavior. As shown in FIG. 7, an organic bistable material has two current-voltage characteristics of a high resistance state 51 (off state) and a low resistance state 52 (on state), and exhibits a non-linear response property. Preliminarily applying a bias voltage Vb, when a voltage is raised up to Vth2 (upper threshold voltage) or higher, a transition from off-state to on-state occurs, and when the voltage is lowered down to Vth1 (lower threshold voltage) or lower, a transition from on-state to off-state occurs, changing the resistance value of the material. A so-called switching action can be performed by applying a voltage at Vth2 or higher or a voltage at Vth1 or lower to the organic bistable material. The Vth1 and Vth2 can be voltages in pulse form.
Known organic bistable materials exhibiting non-linear response include various types of organic complexes. For example, R. S. Potember et al. produced a switching device having two stable resistance values for a voltage using Cu-TCNQ (copper—tetracyanoquinodimethane) complex (R. S. Potember et al., Appl. Phys. Lett., vol. 34, p. 405 (1979)). Kumai et al. observed switching behavior caused by non-linear response using a single crystal of K-TCNQ (potassium—tetracyanoquinodimethane) complex (Kumagai et al., Kotai Buturi (Solid State Physics), vol. 35, p. 35 (2000)). Adachi et al. formed a thin film of Cu-TCNQ complex by means of vacuum evaporation, and revealing its switching characteristic, studied the feasibility of applying it to an organic EL matrix (Adachi et al., Abstract of 2002 Spring Meeting of the Japanese Society of Applied Physics, third volume, p. 1236).
The switching devices using an organic charge transfer complex mentioned above, however, had the problems described in the following. The organic bistable material, being a charge transfer complex, is a two-component material consisting of a molecule or a metallic element exhibiting a donor characteristic and a molecule such as TCNQ exhibiting an acceptor characteristic.
Consequently, fabrication of a switching device requires strict control of the composition ratio of the two components. In the charge transfer complex of a two-component system, as shown in FIG. 8, each of the donor molecules and the acceptor molecules laminates in a columnar shape forming donor molecule column 61 and acceptor molecule column 62, and the components of each column perform partial charge transfer between the molecules (or metal atoms) exhibiting bistability. Therefore, any excess or deficiency in the proportion of the two components critically affects the bistable performance of the device as a whole.
In the Cu-TCNQ complex mentioned above, for example, different proportions between Cu and TCNQ bring about different crystallinity and electrical characteristics and cause variation in bistable performance. It is particularly difficult to obtain a large area and uniform film when using film deposition by means of vacuum evaporation, due to differences in vapor pressure of the two components and, in the case of co-evaporation, geometric arrangement of the individual evaporation source for each material. Therefore, it is difficult to mass produce switching devices that perform with a steady bistability characteristic and uniform quality from the conventional organic bistable materials of two-component systems.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.